For certain types of minimally invasive medical procedures, real time information regarding the condition of the treatment site within the body is unavailable. This lack of information inhibits the clinician when employing catheter to perform a procedure. An example of such procedures is tumor and disease treatment in the liver and prostate. Yet another example of such a procedure is surgical ablation used to treat atrial fibrillation. This condition in the heart causes abnormal electrical signals, known as cardiac arrhythmias, to be generated in the endocardial tissue resulting in irregular beating of the heart.
The most frequent cause of cardiac arrhythmias is an abnormal routing of electricity through the cardiac tissue. In general, most arrhythmias are treated by ablating suspected centers of this electrical misfiring, thereby causing these centers to become inactive. Successful treatment, then, depends on the location of the ablation within the heart as well as the lesion itself. For example, when treating atrial fibrillation, an ablation catheter is maneuvered into the right or left atrium where it is used to create ablation lesions in the heart. These lesions are intended to stop the irregular beating of the heart by creating non-conductive barriers between regions of the atria that halt passage through the heart of the abnormal electrical activity.
The lesion should be created such that electrical conductivity is halted in the localized region (transmurality), but care should be taken to prevent ablating adjacent tissues. Moreover, because the ablation process can raise tissue temperature due to resistive heating, excessive heating of the tissue can cause undesirable charring and localized coagulation, and even evaporate water in the blood and tissue leading to steam pops which can damage tissue.
Thus, it would be desirable to provide an electrophysiologic catheter that permits real time monitoring of tissue temperature during ablation and lesion formation to prevent, or at least minimize, critical thresholds in temperature associated with such events as steam pop, thrombus formation, char, etc. Because all tissues emit black body radiation that is directly related to temperature, it would be desirable for an electrophysiologic catheter to detect black body radiation for noninvasive temperature determination.
A black body radiation curve such as in FIG. 1 shows that the black body radiates energy at every wavelength (the curve approaches the x-axis but never touches it). The black body has a wavelength at which most of the radiant energy is emitted, and in FIG. 1, the peak wavelength is about 500 nm for a temperature of 5000K. This peak wavelength, along with the radiation curve, however varies with temperature, as shown in FIG. 2. In particular, as the temperature increases, the peak wavelength decreases, as well as the standard amount of energy emitted by the black body, as represented by the area under each curve.
Black body laws can be applied to many thing, including the human body. Much of a person's energy is lost in the form of electromagnetic radiation, of which most is infrared. A human body has a temperature is about 36.5 C (98.6 F or 310 K) and infrared (IR) radiation is of a wavelength longer than that of visible light but shorter than that of radio waves. Infrared radiation spans three orders of magnitude and has wavelengths between approximately 750 nm and 1 mm. As such, the peak wavelength for human tissue may range between about 2000 nm and 4000 nm, preferably between about 2000 nm and 3100 nm, and more preferably between about 2000 nm and 3000 nm.
Therefore, by monitoring the peak wavelength or peak wavelength region(s) of the black body radiation of tissue, the temperature of the tissue can be obtained in real time as a means of preventing overheating of tissue during ablation and lesion formation.